John Marsden’s superlative “Tomorrow” series set the tone for young adult political/social thrillers. Now there are two new adventures: Emmy Laybourne’s “Monument 14,” set in a slightly futuristic Monument, Colorado; and “No Safety In Numbers” by Dayna Lorentz.

In “Monument 14,” Labourne’s debut novel, an apocalyptic event strands 14 school children in a Target-like megastore. The crisis reshapes the social dynamics of the older students, not quite reversing the pecking order but definitely setting the popular jock and the bully back on their heels.

With one foot in childhood — yay! Sundaes for breakfast — and the other in adulthood — upon realizing that the store’s air conditioning system is sucking in noxious bio-weapon chemicals, the kids figure out how to seal off the outside air intake — “Monument 14″ is unsettlingly plausible, or plausibly unsettling. (A couple of kids takes advantage of the store pharmacy and liquor departments, but other kids prove to be surprisingly mature and pragmatic.) It’s a fast, taut thriller for readers aged 13 and up.